Importance of the combined effects of dissolved oxygen and pH on optimization of nitrogen removal in anammox-enriched granular sludge

Research progress of anaerobic ammonium oxidation (Anammox) process based on integrated fixed-film activated sludge (IFAS)

The integrated fixed-film activated sludge (IFAS) process is considered one of the cutting-edge solutions to the traditional wastewater treatment challenges, allowing suspended sludge and attached biofilm to grow in the same system. In addition, the coupling of IFAS with anaerobic ammonium oxidation (Anammox) can further improve the efficiency of biological denitrification. This paper summarises the research progress of IFAS coupled with the anammox process, including partial nitrification anammox, simultaneous partial nitrification anammox and denitrification, and partial denitrification anammox technologies, and describes the factors that limit the development of related processes. The effects of dissolved oxygen, influent carbon source, sludge retention time, temperature, microbial community, and nitrite-oxidising bacteria inhibition methods on the anammox of IFAS are presented. At the same time, this paper gives an outlook on future research focus and engineering practice direction of the process.

Effects of zinc ion concentrations on the performance of SBR treating livestock wastewater and analysis of microbial community

Zinc ion (Zn2+) is a frequently occurring heavy metal in livestock wastewater. The effects of Zn2+ on the physicochemical properties and the microbial distribution of activated sludge are essential to controlling nitrogen removal performance. Nevertheless, there are raw studies on the effects of Zn2+ on nitrogen removal. This study investigated the effect of Zn2+ on the treatment performance of livestock wastewater in a sequencing batch reactor (SBR). The results indicated the low Zn2+ concentrations could improve nitrogen removal performance. However, as the Zn2+ concentration increased, the total nitrogen (TN) removal performance of the reactor gradually deteriorated. When the Zn2+ concentration was 90.00 mg/L, the TN removal efficiency was the lowest, only 2.40%. The contents of the Extracellular polymeric substance (EPS) presented a trend of first increasing and then decreasing with the increase of Zn2+ concentration, and the main reason was the decrease of protein-like and tryptophan-like. The 16SrRNA analysis indicated that Zn2+ within a specific concentration could increase the operational taxonomic units (OTUs) number, microbial richness, and diversity of microorganisms in the SBR. However, with Zn2+ concentration exceeding 10.00 mg/L, the relative abundance of denitrification functional bacteria (Dechloromonas, Nitrospira, and Thauera) decreased.

Adaptation of anammox process for nitrogen removal from acidic nitritation effluent in a low pH moving bed biofilm reactor

Acidic partial nitritation (PN) has emerged to be a promisingly stable process in wastewater treatment, which can simultaneously achieve nitrite accumulation and about half of ammonium reduction. However, directly applying anaerobic ammonium oxidation (anammox) process to treat the acidic PN effluent (pH 4-5) is susceptible to the inhibition of anammox bacteria. Here, this study demonstrated the adaptation of anammox process to acidic pH in a moving bed biofilm reactor (MBBR). By feeding the laboratory-scale MBBR with acidic PN effluent (pH = 4.6 ± 0.2), the pH of an anammox reactor was self-sustained in the range of pH 5 - 6. Yet, a high total nitrogen removal efficiency of over 80% at a practical loading rate of up to 149.7 ± 3.9 mg N/L/d was achieved. Comprehensive microbial assessment, including amplicon sequencing, metagenomics, cryosection-FISH, and qPCR, identified that Candidatus Brocadia, close to known neutrophilic members, was the dominant anammox bacteria. Anammox bacteria were found present in the inner layer of thick biofilms but barely present in the surface layer of thick biofilms and in thin biofilms. Results from batch tests also showed that the activity of anammox biofilms could be maintained when subjected to pH 5 at a nitrite concentration of 10 mg N/L, whereas the activity was completely inhibited after disturbing the biofilm structure. These results collectively indicate that the anammox bacteria enriched in the present acidic MBBR could not be inherently acid-tolerant. Instead, the achieved stable anammox performance under the acidic condition is likely due to biofilm stratification and protection. This result highlights the biofilm configuration as a useful solution to address nitrogen removal from acidic PN effluent, and also suggests that biofilm may play a critical role in protecting anammox bacteria found in many acidic nature environments.

A review of anammox metabolic response to environmental factors: Characteristics and mechanisms

Anaerobic ammonium oxidation (anammox) is a promising low carbon and economic biological nitrogen removal technology. Considering the anammox technology has been easily restricted by environmental factors in practical engineering applications, therefore, it is necessary to understand the metabolic response characteristics of anammox bacteria to different environmental factors, and then guide the application of the anammox process. This review presented the latest advances of the research progress of the effects of different environmental factors on the metabolic pathway of anammox bacteria. The effects as well as mechanisms of conventional environmental factors and emerging pollutants on the anammox metabolic processes were summarized. Also, the role of quorum sensing (QS) mediating the bacteria growth, gene expression and other metabolic process in the anammox system were also reviewed. Finally, interaction and cross-feeding mechanisms of microbial communities in the anammox system were discussed. This review systematically summarized the variations of metabolic mechanism response to the external environment and cross-feeding interactions in the anammox process, which would provide an in-depth understanding for the anammox metabolic process and a comprehensive guidance for future anammox-related metabolic studies and engineering applications.

Rapid Start-Up Characteristics of Anammox under Different Inoculation Conditions

The long multiplication time and extremely demanding enrichment environment requirements of Anammox bacteria (AAOB) have led to difficult reactor start-ups and hindered its practical dissemination. Few feasibility studies have been reported on the recovery of AAOB activity initiation after inlet substrate disconnection caused by an unfavorable condition, and few factors, such as indicators of the recovery process, have been explored. Therefore, in this experiment, two modified expanded granular sludge bed reactors (EGSB) were inoculated with 1.5 L anaerobic granular sludge (AGS) + 1 L Anammox sludge (AMS) (R1) and 2.5 L anaerobic granular sludge (AGS) (R2), respectively. After a long-term (140 days) starvation shock at a high temperature (38 °C), the bacteria population activity recovery experiments were conducted. After 160 days, both reactors were successfully started up, and the total nitrogen removal rates exceeded 87%. Due to the experimental period, the total nitrogen removal rate of R2 was slightly higher than that of R1 in the final stage. However, it is undeniable that R2 had a relatively long activity delay during startup, while R1 had no significant activity delay during startup. The sludge obtained from R1 had a higher specific anammox activity (SAA). Analysis of the extracellular polymer substances (EPS) results showed that the extracellular polymer content in R1 was higher than that in R2 throughout the recovery process, indicating that R1 had higher sludge stability and denitrification performance. Scanning electron microscopy (SEM) analysis showed that more extracellular filamentous bacteria could be seen in the R1 reactor with better morphology of Anammox bacteria. In contrast, the R2 reactor had fewer extracellular hyphae and micropores as a percentage and higher filamentous bacteria content. The results of microbial 16SrDNA analysis showed that R1 used AAOB as inoculum to initiate Anammox, and the reactor was enriched with Anammox bacteria earlier and in much greater abundance than R2. The experimental results indicated that inoculating mixed anaerobic granular sludge and Anammox sludge to initiate an anammox reactor was more effective.

A quantified nitrogen metabolic network by reaction kinetics and mathematical model in a single-stage microaerobic system treating low COD/TN wastewater

A single-stage intermittent aeration microaerobic reactor (IAMR) has been developed for the cost-effective nitrogen removal from piggery wastewater with a low ratio of chemical oxygen demand (COD) to total nitrogen (TN). In this study, a quantified nitrogen metabolic network was constructed based on the metagenomics, reaction kinetics and mathematical model to provide a revealing insight into the nitrogen removal mechanism in the IAMR. Metagenomics revealed that a complex nitrogen metabolic network, including aerobic ammonia and nitrite oxidation, anammox, denitrification via nitrate and nitrite, and nitrate respiration, existed in the IAMR. A novel method for solving kinetic parameters with high stability was developed based on a genetic algorithm. Use this method to calculate the kinetics of various reactions involved in nitrogen metabolism. Kinetics revealed that simultaneous partial nitritation-anammox (PN/A) and partial denitrification-anammox (PDN/A) were the dominant approaches to nitrogen removal in the IAMR. Finally, a kinetics-based model was proposed for quantitatively describing the nitrogen metabolic network under the limitation of COD. 58% ∼ 67% of nitrogen was removed via the anammox-based processes (PN/A and PDN/A), but only 7% ∼ 12% and 1% ∼ 2% of nitrogen were removed via heterotrophic denitrification of nitrite and nitrate, respectively. The half-inhibition constant of dissolved oxygen (DO) on anammox was simulated as 0.37 ∼ 0.60 mg L^-1, filling the gap in quantifying DO inhibition on anammox. High-frequency intermittent aeration was identified as the crucial measure to suppress nitrite-oxidizing bacteria, although it has a high affinity for DO and NO₂^--N. In continuous aeration mode, the simulated NO₃^--N in the IAMR would rise by 39.6%. The research provides a novel insight into the nitrogen removal mechanism in single-stage microaerobic systems and provides a reliable approach to practicing PN/A and PDN/A for cost-effective nitrogen removal.

Evaluation of long-term preservation and reactivation efficiency of anaerobic ammonium oxidation (anammox) microorganisms based on activation energy

The preservation efficiency of mainstream (M-ANA) and sidestream anaerobic ammonium oxidation (anammox) (S-ANA) were evaluated based on their activation energy (E_a). The E_a of M-ANA cultivated under low nitrogen loads was lower than that of S-ANA, which greatly contributed to enhancing the viability of anammox during preservation at 4 °C. After preservation for 140 d, the decay rate (b_AN) of M-ANA ranged from 0.0012 to 0.0013/d; the b_AN of S-ANA was 0.0036-0.0041/d. The addition of hydrazine, which requires minimal energy to activate anammox metabolism, is highly beneficial for the viability of microorganisms. The low E_a of anammox contributes to efficient reactivation with rapid reactivation of heme c, and the addition of hydrazine makes the process more beneficial. Although the specific nitrogen removal rate of the M-SNA seed sludge was much lower than that of S-ANA, the rate of M-ANA became higher after 48 days of reactivation.

A comprehensive root cause analysis of anammox bioreactor performance decline

The cultivation of anaerobic ammonia oxidizing bacteria (anammox) has gained enormous awareness over the last few decades. Although numerous studies focus massively on successfully growing these anammox to different enrichment environments, in reality, the failure rates are somewhat comparable to the reported success rates. This study combines a variety of measurement techniques to observe and monitor the sequence of a bioreactor performance decline following elevated influent substrate concentration. After attaining stable substrate removal throughout a nitrogen loading rate (NLR) range of 0.691 to 1.669 kg-N·m^-3·d^-1, the performance of the lab-scale anammox-sequencing batch reactor (SBR) abruptly broke down as the NLR reached 2.01 kg-N·m^-3·d^-1. The gathered information showed that the increased NLR firstly caused a significant and unfavorable change in the free ammonia (FA) and free nitrous acid (FNA) concentration in the bioreactor. A subsequent drop in N₂ production and a decline from a peak high of 0.381 to a low of 0.012 kg-N·kg-VSS^-3·d^-1 of the specific nitrogen removal rate (SNRR) led to an 82% absurd decline in microbial cellular energy production. Prior to these anammox switching to survival mode and secreting larger quantities (32% higher) of extracellular polymeric substances (EPS), the activity of syntrophic decomposers increased substantially leading to the internal production of excess CO₂ in the bioreactor and thereby diverging the bioreactor pH to lower levels. The purposes of this study are to understand the reason an anammox process shows different signals during a decline phase and to enable immediate response to performance deterioration.

Comparison of the Efficiency of Deammonification under Different DO Concentrations in a Laboratory-Scale Sequencing Batch Reactor

The efficiency of deammonification depends on the cooperation of ammonium oxidizing bacteria and archaea (AOB/AOA), anaerobic ammonium oxidizing bacteria (AnAOB) and the effective suppression of nitrite oxidizing bacteria (NOB) that compete with AnAOB for nitrite (NO2-N). One of the effective NOB suppression strategies is intermittent aeration. However, it is important to have a good understanding of the optimum dissolved oxygen (DO) value in the aeration period and optimize the non-aeration time used during the reaction phase. This study comprised the investigation of the effect of different DO set points (0.4, 0.7, 1.0 and 1.5 mg O2/L) under the same aeration length off/on (12/3 min). Moreover, three different intermittent aeration modes (9/3, 6/3, 3/3) under the same DO set point (0.7 mg O2/L) were more investigated. The experiment was conducted for 6 months (180 days) in a laboratory-scale sequencing batch reactor (SBR) with a working volume of 10 L. The results indicated that a high N removal efficiency was achieved 74% at the DO set point = 0.7 mg O2/L during aeration strategy off/on (6/3 min) due to the low nitrate production rate (NPR) 0.9 mg N/g VSS/h and high ammonium utilization rate (AUR) 13 mg N/g VSS/h (NPR/AUR = 0.06). Mathematical modeling results confirmed that the feasible DO set point 0.7 and intermittent aeration mode off/on (6/3 min) were especially suitable for the optimal balance between the NOB suppression and keeping high activities of AOB and anammox in the system.

Improving the resistance of Anammox granules to extreme pH shock: The effects of denitrification sludge EPS enhanced by a fluctuating C/N ratio cultivation on granules

This study investigated the effects of denitrification sludge EPS enhanced (DS-EPS_CN) by a fluctuating carbon and nitrogen ratio (C/N) cultivation strategy on the properties of Anammox granules under extreme acid or alkaline shock. The results showed that the DS-EPS_CN significantly improved the nitrogen removal performance of low-density Anammox granular sludge (Granules-_L) and high-density Anammox granular sludge (Granules_-H) under extreme acid shock (pH 5.0). The contents of high-molecular-weight substances (such as aromatic proteins and polysaccharides) in the DS-EPS_CN rose markedly, contributing to a substantial increase in the flocculation efficiency under acidic conditions and increasing the granule stability. In addition, abundant amounts of N-butyryl-dl-homoserine lactone (C4-HSL) and N-hexanoyl-dl-homoserine lactone (C6-HSL) in the DS-EPS_CN promoted the granule activity. However, under extreme alkaline shock (pH 10.5), the flocculation efficiency of the DS-EPS_CN was poor, and the addition of DS-EPS_CN had no influence on the stability of the granules but improved the activity of the Granules_-H. The reason was that the release mechanism of the endogenous acyl-homoserine lactone (AHL) signals in the Granules_-H was activated by the exogenous C4-HSL and C6-HSL in the DS-EPS_CN under alkaline conditions, leading to increased Granules_-H activity. This research provides a novel approach to enhance the resistance of Anammox granular sludge to extreme pH shock.

The Influence of External Load on the Performance of Microbial Fuel Cells

In this work, the effect of the external load on the current and power generation, as well as on the pollutant removal by microbial fuel cells (MFCs), has been studied by step-wise modifying the external load. The load changes included a direct scan, in which the external resistance was increased from 120 Ω to 3300 Ω, and a subsequent reverse scan, in which the external resistance was decreased back to 120 Ω. The reduction in the current, experienced when increasing the external resistance, was maintained even in the reverse scan when the external resistance was step-wise decreased. Regarding the power exerted, when the external resistance was increased below the value of the internal resistance, an enhancement in the power exerted was observed. However, when operating near the value of the internal resistance, a stable power exerted of about 1.6 µW was reached. These current and power responses can be explained by the change in population distribution, which shifts to a more fermentative than electrogenic culture, as was confirmed by the population analyses. Regarding the pollutant removal, the effluent chemical oxygen demand (COD) decreased when the external resistance increased up to the internal resistance value. However, the effluent COD increased when the external resistance was higher than the internal resistance. This behavior was maintained in the reverse scan, which confirmed the modification in the microbial population of the MFC.

Effective nitrogen removal from ammonium-depleted wastewater by partial nitritation and anammox immobilized in granular and thin layer gel carriers

This study investigates the effect of physicochemical conditions on the partial nitritation and anammox treatment by immobilized ammonia oxidizers under ammonium-deplete conditions. The impact of oxygen and temperature was studied by measuring the activity of immobilized aerobic and anaerobic ammonia-oxidizing organisms (Ammonia-oxidizing bacteria (AOB) and archaea (AOA), and Anammox bacteria) embedded in polyvinyl alcohol - sodium alginate (PVA-SA) beads and in thin layer poly-ethylene glycol hydrogels. Beads and flat hydrogels were incubated in a fluidized bed reactor (FBR) and in two flow cells, respectively. Both systems were fed with synthetic wastewater (15 mg N-NH₄⁺/L) at different temperatures (20 °C and/or 30 °C) and different dissolved oxygen (DO) concentrations (0.1, 0.3, 0.5 and/or 1 mg/L) over 152 and 207 days, respectively. The FBR system had a maximum removal rate of 1.7 g-N/m³/d at 0.1 mg O₂/L, corresponding to 80% removal efficiency, while a high aerobic ammonia-oxidizing activity but a partial oxygen inhibition of Anammox bacteria were observed at higher DO concentrations. In both flow cells, nitrogen removal efficiency was highest (80%) at 30 °C and 1 mg O₂/L while removal was less favorable at lower DO and lower temperature. Our results indicate a potential use of hydrogel beads for an energy efficient technology with reduced aeration demand for treating low ammonia wastewater, while layered hydrogels are a possible first step for biological treatments of wastewater using tangential flow. In addition, we provide blueprint drawings of the flow cells, which may be used to 3D-print the apparatus for other applications.

Domestic Sewage Treatment Using a One-Stage ANAMMOX Process

A one-stage anaerobic ammonium oxidation (ANAMMOX) reactor can be quickly started within 40 days by mixing partial nitrifying sludge with ANAMMOX granular sludge with an average temperature of 30 °C. After 70 days of nitrogen load acclimation, Acinetobacter, including Candidatus Kuenenia, became the dominant strain of the system within the reactor, which exhibited high efficiency and a stable nitrogen removal performance. At an influent chemical oxygen demand (COD), NH₄⁺-N content, total nitrogen (TN) content, hydraulic retention time (HRT), temperature, and reactor dissolved oxygen (DO) content of 100, 60, and 70 mg/L, 6 h, 30 ± 1 °C, and below 0.6 mg/L, respectively, the one-stage ANAMMOX reactor could effectively treat domestic sewage on campus. The removal rates of COD, NH₄⁺-N, and TN were approximately 89%, 96.7%, and 70%, respectively.

Anammox and partial denitrification coupling: a review

As a new wastewater biological nitrogen removal process, anammox and partial denitrification coupling not only plays a significant role in the nitrogen cycle, but also holds high engineering application value. Because anammox and some denitrifying bacteria are coupled under harsh living conditions, certain operating conditions and mechanisms of the coupling process are not clear; thus, it is more difficult to control the process, which is why the process has not been widely applied. This paper analyzes the research focusing on the coupling process in recent years, including anammox and partial denitrification coupling process inhibitors such as nitrogen (NH₄⁺, NO₂⁻), organics (toxic and non-toxic organics), and salts. The mechanism of substrate removal in anammox and partial denitrification coupling nitrogen removal is described in detail. Due to the differences in process methods, experimental conditions, and sludge choices between the rapid start-up and stable operation stages of the reactor, there are significant differences in substrate inhibition. Multiple process parameters (such as pH, temperature, dissolved oxygen, redox potential, carbon-to-nitrogen ratio, and sludge) can be adjusted to improve the coupling of anammox and partial denitrification to modify nitrogen removal performance.

As a new wastewater biological nitrogen removal process, anammox and partial denitrification coupling not only plays a significant role in the nitrogen cycle, but also holds high engineering application value.

Effect of operating conditions on N2O production in an anammox sequencing batch reactor containing granular sludge

Nitrous oxide (N₂O) is one of the gases with the greatest impact in the atmosphere due to its persistence and significant contribution to the greenhouse effect. This study provides an insight into the dynamics of N₂O production in wastewater nitrogen removal systems. A 10 L sequencing batch reactor containing enriched anammox biomass was subjected to different operational conditions, i.e., temperature, feed time, NO₂ ^-/NH₄ ⁺ ratio and the initial concentrations of NH₄ ⁺ and NO₂ ^-. Tests showed no significant differences in maximum N₂O production when the system was operated with a shorter feed time and no increase in the operating temperature. A higher N₂O production was observed when the initial NO₂ ^-/NH₄ ⁺ ratio increased from 1.3 to 1.7 and 1.9. The highest initial concentration of NO₂ ^- was linked to an increase in residual N₂O at the end of the batch cycle, probably due to heterotrophic denitrifying metabolism.

Fast start-up of the single-stage nitrogen removal using anammox and partial nitritation (SNAP) from conventional activated sludge in a membrane-aerated biofilm reactor

The single-stage nitrogen removal using anammox and partial nitritation (SNAP) is a promising alternative for low-cost ammonium removal from wastewaters. This study aimed to evaluate the anammox biomass enrichment and SNAP process start-up in a laboratory-scale membrane-aerated biofilm reactor (MABR) at nitrogen loading rates of 50 g N.m^-3.d^-1 (period 1) and 100 g N.m^-3.d^-1 (period 2). Anammox activity was observed after 48 days, and the SNAP process was stable after 80 days. In period 1, the average total nitrogen (TN) removal was 78 ± 6%, and the maximum removal was 84%. In period 2, the average TN removal was 61 ± 5%, and the maximum was 69%. Higher dissolved oxygen levels may have caused imbalances in the microbial community in period 2, decreasing the reactor performance. These results demonstrated the potential of the MABR for the fast implementation of the single-stage partial nitritation and anammox processes.

Short-term effects of nanoscale Zero-Valent Iron (nZVI) and hydraulic shock during high-rate anammox wastewater treatment

The stability and resilience of an anaerobic ammonium oxidation (anammox) system under transient nanoscale Zero-Valent Iron (nZVI) (50, 75 and 100 mg L^-1), hydraulic shock (2-fold increase in flow rate) and their combination were studied in an up-flow anaerobic sludge blanket reactor. The response to the shock loads can be divided into three phases i.e. shock, inertial and recovery periods. The effects of the shock loads were directly proportional to the shock intensity. The effluent quality was gradually deteriorated after exposure to high nZVI level (100 mg L^-1) for 2 h. The higher effluent sensitivity index and response caused by unit intensity of shock was observed under hydraulic and combined shocks. Notably, the specific anammox activity and the content of heme c were considerably reduced during the shock phase and the maximum loss rates were about 30.5% and 24.8%, respectively. Nevertheless, the extracellular polymeric substance amount in the shock phase was enhanced in varying degrees and variation tendency was disparate at all the tested shock loads. These results suggested that robustness of the anammox system was dependent on the magnitude shocks applied and the reactor resistance can be improved by reducing hydraulic retention time with the increase of nZVI concentration under these circumstances.

Model-Based Evaluation of N2O Production Pathways in the Anammox-Enriched Granular Sludge Cultivated in a Sequencing Batch Reactor

A mechanistic model was developed as an extension of the Activated Sludge Model No. 1 to describe three nitrous oxide (N₂O) production pathways in a laboratory-scale anammox-enriched granular sequencing batch reactor. Heterotrophic denitrification and two processes mediated by ammonia oxidizing bacteria (AOB), that is, ammonia (NH₄⁺) oxidation via hydroxylamine (NH₂OH) and autotrophic denitrification, were considered. A systematic model calibration and validation protocol was developed to obtain a unique set of kinetic parameters in the extended model. The dynamic nitrate (NO₃^-), nitrite (NO₂^-), NH₄⁺ and N₂O behaviors were accurately predicted (R² ≥ 0.81) under five different nitrogen loading conditions. The predicted N₂O production factor ranged from 1.7 to 2.9%. The model-based analysis also revealed the dominant N₂O production mechanisms in terms of the actual process conditions, that is, NH₄⁺ oxidation via NH₂OH when only NH₄⁺ was supplied, heterotrophic denitrification when only NO₂^- was supplied, and a shift of the dominant mechanism when a mixture of NH₄⁺ and NO₂^- was supplied.

Nitrogen removal via a single-stage PN-Anammox process in a novel combined biofilm reactor

This study developed a partial nitrification (PN) and anaerobic ammonia oxidation (Anammox) process for treating high-ammonia wastewater using an innovative biofilm system in which ammonia oxidizing bacteria grew on fluidized Kaldnes (K1) carriers and Anammox bacteria grew on fixed acryl resin carriers. The airlift loop biofilm reactor (ALBR) was stably operated for more than 4 months under the following conditions: 35 ± 2 °C, pH 7.5-8.0 and dissolved oxygen (DO) of 0.5-3.5 mg/L. The results showed that the total nitrogen removal efficiency reached a maximum of 75% and the total nitrogen removal loading rate was above 0.4 kg/(d·m³). DO was the most efficient control parameter in the mixed biofilm system, and values below 1.5 mg/L were observed in the riser zone for the PN reaction, while values below 0.8 mg/L were observed in the downer zone for the Anammox reaction. Scanning electron microscopy and Fluorescence In Situ Hybridization images showed that most of the nitrifying bacteria were distributed on the K1 carriers and most of the Anammox bacteria were distributed within the acryl resin carriers. Therefore, the results indicate that the proposed combined biofilm system is easy to operate and efficient for the treatment of high-ammonia wastewater.

Modeling the pH effects on nitrogen removal in the anammox-enriched granular sludge

The aim of the study was to determine the pH effects on nitrogen removal in the anammox-enriched granular sludge. The experimental data were extracted from a 4 L completely-mixed batch reactor with the granular sludge at different initial pH values (6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5) and constant temperature T = 30 °C. Simulations were run in GPS-X 6.4 using a comprehensive mechanistic model Mantis2. Two kinetic parameters, the maximum specific growth rates of ammonia oxidizing bacteria (AOB) and anammox bacteria, were optimized at different pH scenarios. The inhibitory effects of the pH extremes on the anammox-enriched sludge were discussed in terms of the inhibition of free nitrous acid and free ammonia and metabolic mechanisms. Two different pH functions were used to examine the pH effects on the nitrogen removal kinetics. The pH optima for AOB and anammox bacteria were 7.4 and 7.6, respectively. The maximum specific growth rates of AOB and anammox bacteria at the pH optima were 0.81-0.85 d^-1 and 0.36-0.38 d^-1 (at T = 30 °C). The measured specific anammox activities (SAAs), predicted SAAs by Mantis2 and fitted SAAs by the Michaelis pH function at the pH optima were 0.895, 0.858 and 0.831 gN/(gVSS·d), respectively (VSS: volatile suspended solids).